Re: [PATCH v2] drivers/perf: arm_spe: Fix consistency of SYS_PMSCR_EL1.CX

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 7/13/22 15:36, Suzuki K Poulose wrote:
> On 13/07/2022 09:59, Anshuman Khandual wrote:
>> The arm_spe_pmu driver will enable SYS_PMSCR_EL1.CX in order to add CONTEXT
>> packets into the traces, if the owner of the perf event runs with required
>> capabilities i.e CAP_PERFMON or CAP_SYS_ADMIN via perfmon_capable() helper.
>>
>> The value of this bit is computed in the arm_spe_event_to_pmscr() function
>> but the check for capabilities happens in the pmu event init callback i.e
>> arm_spe_pmu_event_init(). This suggests that the value of the CX bit should
>> remain consistent for the duration of the perf session.
>>
>> However, the function arm_spe_event_to_pmscr() may be called later during
>> the event start callback i.e arm_spe_pmu_start() when the "current" process
>> is not the owner of the perf session, hence the CX bit setting is currently
>> not consistent.
>>
>> One way to fix this, is by caching the required value of the CX bit during
>> the initialization of the PMU event, so that it remains consistent for the
>> duration of the session. It uses currently unused 'event->hw.flags' element
>> to cache perfmon_capable() value, which can be referred during event start
>> callback to compute SYS_PMSCR_EL1.CX. This ensures consistent availability
>> of context packets in the trace as per event owner capabilities.
>>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Fixes: cea7d0d4a59b ("drivers/perf: Open access for CAP_PERFMON privileged process")
>> Reported-by: German Gomez <german.gomez@arm.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> Changes in V2:
>>
>> - Moved CONFIG_PID_IN_CONTEXTIDR config check inside the helper per Suzuki
>> - Changed the comment per Suzuki
>> - Renamed the helpers Per Suzuki
>> - Added "Fixes: " tag per German
>>
>> Changes in V1:
>>
>> https://lore.kernel.org/all/20220712051404.2546851-1-anshuman.khandual@arm.com/
>>
>>   drivers/perf/arm_spe_pmu.c | 23 +++++++++++++++++++++--
>>   1 file changed, 21 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
>> index db670b265897..c4290b0492fd 100644
>> --- a/drivers/perf/arm_spe_pmu.c
>> +++ b/drivers/perf/arm_spe_pmu.c
>> @@ -39,6 +39,24 @@
>>   #include <asm/mmu.h>
>>   #include <asm/sysreg.h>
>>   +/*
>> + * Cache if the event is allowed to trace Context information.
>> + * This allows us to perform the check, i.e, perfmon_capable(),
>> + * in the context of the event owner, once, during the event_init().
>> + */
>> +#define SPE_PMU_HW_FLAGS_CX            BIT(0)
>> +
>> +static void set_spe_event_has_cx(struct perf_event *event)
>> +{
>> +    if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR) && perfmon_capable())
>> +        event->hw.flags |= SPE_PMU_HW_FLAGS_CX;
>> +}
>> +
>> +static bool get_spe_event_has_cx(struct perf_event *event)
>> +{
>> +    return !!(event->hw.flags & SPE_PMU_HW_FLAGS_CX);
>> +}
>> +
>>   #define ARM_SPE_BUF_PAD_BYTE            0
>>     struct arm_spe_pmu_buf {
>> @@ -272,7 +290,7 @@ static u64 arm_spe_event_to_pmscr(struct perf_event *event)
>>       if (!attr->exclude_kernel)
>>           reg |= BIT(SYS_PMSCR_EL1_E1SPE_SHIFT);
>>   -    if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR) && perfmon_capable())
>> +    if (get_spe_event_has_cx(event))
>>           reg |= BIT(SYS_PMSCR_EL1_CX_SHIFT);
>>         return reg;
>> @@ -710,7 +728,8 @@ static int arm_spe_pmu_event_init(struct perf_event *event)
>>           return -EOPNOTSUPP;
>>   
> 
> 
> 
>>       reg = arm_spe_event_to_pmscr(event);
>> -    if (!perfmon_capable() &&
>> +    set_spe_event_has_cx(event);
> 
> This seems to be wrong. We need to set the event_has_cx() *before*
> we call arm_spe_event_to_pmscr(), as the latter uses
> get_spe_event_has_cx().

Right, the order needs to be reversed.

> 
>> +    if (!get_spe_event_has_cx(event) &&
>>           (reg & (BIT(SYS_PMSCR_EL1_PA_SHIFT) |
> 
> And we must retain the perfmon_capable() check here to ensure that any of the following options are usable without CX. e.g,
> if CONFIG_PID_IN_CONTEXTIDR is not enabled, !get_spe_event_has_cx() doesn't imply !perfmon_capable().

Right, but BIT(SYS_PMSCR_EL1_CX_SHIFT) check could be dropped here because
that could not have been set in function arm_spe_event_to_pmscr() without
perfmon_capable() ?

> 
> 
>>               BIT(SYS_PMSCR_EL1_CX_SHIFT) |
>>               BIT(SYS_PMSCR_EL1_PCT_SHIFT))))
> 
> Suzuki